1. Field of the Invention
The present invention relates to a switched resonator and more particularly to a switched resonator formed, for example, from a capacitor and transmission lines configured with a plurality of RF switches to provide different inductances, the transmission lines being electrically connected to the capacitor forming a resonator with switchable resonant frequencies.
2. Description of the Prior Art
Heterojunction bipolar transistor (HBT) device technology is known for low phase noise VCO applications due to the 1/f noise of such devices relative to MESFETs at microwave to millimeter-wave cut off frequencies. Known HBT VCO devices have superior phase noise performance compared to MESFET VCOs and comparable performance to silicon based VCO's at the C-Ku-band frequencies. However, above such frequencies, HBT technology is preferred for low phase noise VCO applications because of the superior frequency operating capability of HBT based devices compared to the silicon based BJTs and lower device phase noise compared to MESFETs and HEMT technologies. However, for relatively high performance frequency synthesizer applications, phase noise performance over a relatively wide tuning bandwidth provides relatively unacceptable performance. In order to satisfy the low phase noise and frequency tuning bandwidth requirements, often several VCO's are used which cover several narrower overlapping tuning bandwidths forming a switched band synthesizer. The narrower VCO tuning bandwidths provide a higher Q in order to achieve lower phase noise.
A schematic diagram of a known frequency synthesizer is Ad illustrated in FIG. 1 and generally identified with the reference number 100. The frequency synthesizer 100 is a four-band synthesizer and includes 4 voltage controlled oscillators (VCO) 102, 104, 106 and 108. Each of the VCO's 102, 104, 106 and 108 is coupled to a buffer amp 110, 112, 114 and 116, respectively, whose outputs, in turn, are coupled to a switch 118, for example, a single pole, four-throw switch. A plurality of off-chip resonators 120, 122, 124 and 126 are coupled to the inputs of the VCO's 102, 104, 106 and 108, respectively. In order to provide different frequency bands, each of the resonators 102, 120, 122, 124 and 126 are configured to provide different resonator frequencies.
A Quelch signal is used to select among the VCO's 102, 104, 106 and 108. As mentioned above, the VCO's 102, 104, 106 and 108 are connected to the switch 118 by way of the buffer amps 110, 112, 114 and 116 respectively. The Quelch signal is used to select a frequency band and connect the output of the VCO 102, 104, 106 and 108 for the selected bands at the switch 118 to an output amp 128.
Unfortunately, there are several problems with the configuration of the frequency synthesizer as illustrated in FIG. 1. For example, the overall size of circuitry as illustrated in FIG. 1 is relatively large. In particular, the circuit illustrated in FIG. 1 is typically formed as a microwave monolithic integrated circuit (MMIC) with a chip size of for example, 25,000 .mu.m by 25,000 .mu.m. The relatively large size of the frequency synthesizer MMIC is primarily due to the fact that a separate VCO is required for each frequency band of the synthesizer. Moreover, with such a configuration, since the switching is primarily performed by way of the switch 118 at the output of the VCO's 102, 104, 106 and 108, the settling time of such a configuration is less than optimal in certain applications.